Halfdan was an historical 5th-6th century CE Danish king of the Scylding lineage. However Viktor Rydberg in his Teutonic Mythology Volume 1 gives him a divine parentage:
"Like his father, Halfdan was the fruit of a double fatherhood, a divine and a human. Saxo was aware of this double fatherhood, and relates of his Halfdan Berggram that he, although the son of a human prince, was respected as a son of Thor, and honoured as a god among that people who longest remained heathen; that is to say, the Swedes. In his saga, as told by Saxo, Thor holds his protecting hand over Halfdan like a father his son."
Indeed Rydberg not only claims Halfdan to be a son of Thor but also claims that he is the divine Germanic patriarch Mannus, referred to in Germania 2.2 by Tacitus. He argues that Frigg or Jord is the mother of Mannus' father Tuisto who is "a god brought forth from the earth" (Rives translation of Germania) or "an earth-born god" (Mattingley/Handford translation). Jord (Old Norse 'the earth') is an ancient earth Goddess who is referred to as the mother of Thor in the Eddas. It may be that she is far more ancient than Frigg but as we know it was common practice in our mythology for the functions of older deities to be subsumed by newer ones. If Tuisto is the son of Jord then it is possible that Tuisto may equate with Thor or that is the reasoning of Rydberg. What he does not appear to have considered is that Tuisto and Thor may have been brothers and the interest that Thor shows in Halfdan could be that of an uncle for his nephew which we know to have been a strong and sacred bond amongst our ancestors which rivalled that of father and son.
Rudolf Simek in his Dictionary of Northern Mythology speculates that Tuisto was an hermaphrodite and the manuscript form of this name, Tuisco points to the "same basic meaning". Rydberg points out that like Mannus Halfdan has three sons:
"While Mannus has a son Ingaevo, Halfdan has a stepson Yngve, Inge (Svipdag). The scond son of Mannus is named Hermio. Haldan's son with Groa is called Gudhormr. The second part of this name has, as Jassen has already pointed out, nothing to do with ormr. It may be that the name should be divided Gud-hormr, and that hormr should be referred to Hermio. Mannus' third son is Istaevo. The Celtic scholar Zeuss has connected this name with that of the Gothic (more properly Vandal) heroic race Azdingi, and Grimm has again connected Azdingi with Hazdiggo (Haddingr). Halfdan's third son is in Saxo called Hadingus." (Teutonic Mythology Volume 1, Chapter 25)
In Our Fathers' Godsaga Rydberg refers to Halfdan as:
"Skjold-Borgar and Drott's son, the first Germanic king, regarded as Thor's son and honored with divine respect. He is Svipdag's stepfather, Gudhorm and Hadding's father. Tacitus calls him Mannus."
Now turning to the original sources there are some intriguing references in Saxo Grammaticus' The History of the Danes (English translation by Peter Fisher) to a 'champion' called Thori who fights alongside Haldan (Halfdan):
"After Haldan with his assistance had regained complete strength he summoned Thori, a champion of remarkable talents, and declared war on Erik.
"During the fight Haldan observed his line giving way and therefore clambered with Thori to the top of a cliff strewn with rocks. They prised up these boulders, rolled them down on the enemy drawn up on the slopes below and with their falling weight crushed their opponents' battle-line. Ultimately Haldan's stones achieved the victory he had lost with conventional weapons. For this victorious feat he was named Biargramm, a title which appears to be a compound of 'mountains' and 'fierceness'. For this reason he began to be held in such esteem by the Swedes that he was believed to be the son of great Thor, accorded divine honours by the people and judged worthy of public libations." (Book 7)
Dr Hilda Ellis Davidson in her commentary to Saxo states:
"Herrrmann (p.479) suggested that Saxo found the phrase het a Thor (called on Thor) in his source, and rationalised this into a summons sent to a human ally.
"The term Biargrammus (rock-strong) is not found elsewhere. It would be a suitable title for the god Thor himself, since he shattered rocks with his hammer."
Halfdan is also pictured in Saxo as often fighting with an oak club. This was also the weapon of choice of Hercules who the Germanic peoples equated with Donar, the southern Germanic version of Thor. Donarkeule or Donar Clubs were worn as a protective amulet by the early Germanic tribes, certainly until it was later replaced by the more popular Hammer amulets. The Hammer is a later development of the Axe, all three being symbolic of the Germanic and Indo-Germanic Thunder God. There are several English and Scandinavian folktales that refer to Thor or the 'Devil' (a demonised Thor) throwing rocks down upon his enemies, usually from the top of a mountain. The oak and the mountain are strongly associated with the northern European Thunder God.
"As it happened, he was walking through a tract of shady woodland when he tore up by its roots an oak which blocked his path, and by simply stripping off its branches shaped it into a hefty cudgel" (Book 7)
Oak is of course sacred to the Thunder God and you will note that Halfdan must have possessed supernatural strength to uproot an oak by its roots! On an earlier occasion he also used a club as a weapon:
"Afterwards Haldan was about to do battle with the king's nephew Eric, son of his own uncle Frothi, when he learnt that Erik's champion Hakon had the knack of blunting swords by witchcraft. He therefore fitted iron studs to a gigantic club and made it into a battering instrument, as though its wooden strength would prevail against the power of sorcery." (Book 7)
Iron of course is also sacred to Thor and the metal which his second Hammer was later constructed from. On a third occasion he again uses a club as a weapon:
"He lopped down an oak, shaped it into a club and, having joined combat with twelve single-handed, took their lives." (Book 7)
Earlier in Book 3 of Saxo there is a reference to Thor wielding a club:
"But Thor shattered all their shield-defences with the terrific swings of his club, calling on his enemies to attack him as much as his comrades to support him."
W are told that Hother (Hodr in the Eddas):
"rendered the club useless by lopping off the handle."
This caused the Aesir to lose the battle. In the Skaldskaparmal in the Younger Edda we have an explanation as to why the handle of Thor's Hammer was short. Loki had turned into a fly and had on several occasions bit the dwarf smith Sindri, the latter time on his eyelid which distracted him and caused the handle of the Hammer to be made shorter than desired. In the latter of the three accounts which refer to Haldan fighting with his oak club the later part of the tale suddenly and unaccountably refers to his weapon as being a 'giant hammer':
"With the remainder of his band he then went for Haldan, who smashed him down with the giant hammer and deprived him of life and victory." (Book 7)